A KVM solution for simultaneous control of workstations locally and remotely

I have multiple Windows based workstations that have their own monitor, keyboard and mouse.  They are all on the same LAN.  I want to be able to also put these workstations onto a KVM that allows me to control any of the workstations from a remote separate monitor, keyboard and mouse.  I currently have an ethernet KVM extender which allows me to control one computer remotely while I can simultaneously work on it locally.  I need a solution that will allow me to expand that for multiple workstations to one remote station, with the ability to switch between workstations and control them without preventing usage of those workstations, locally.

To put my application in perspective, I am using it for data acquisition and control systems in a large lab.  Sometimes I need to be on the computers in the lab room and then sometimes I need access to the computers right near the experiment we are working on, but I need access to them all.  I know I could do a VNC like setup, but I do not have a spare computer to use, only the monitor, keyboard and mouse which works well with the KVM extender I currently have.  The closest I came to solving this was using a standard 4-port PS/2 KVM I have, connected my three computers to it and using the KVM extender on the main port to reach the remote monitor.  I also had to use USB mice on the workstations, and I had control of both mice at the same time, but I couldn't get multiple keyboards to work.  The computers didn't like having a PS/2 keyboard and a USB keyboard attached at the same time.  Otherwise, if the keyboards worked, I would have been happy.